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<br />II
<br />'CYANIDE 5
<br />This report briefly reviews the technical lit-
<br />erature on ecological and toxicological aspects of
<br />cyanide, with emphasis on fishery and wildlife re-
<br />sources, and provides recommendations for the
<br />protection oC sensitive species of concern to the
<br />U.S. Fish and Wildlife Service. This accountis part
<br />ofa continuing series ofsynoptic reviews prepared
<br />in response to informational requests from Service
<br />environmental specialists.
<br />Chemical Properties
<br />The chemical speciation of cyanides varies ac-
<br />cording to their source. Specific terms used to de-
<br />scribe cyanide include free cyanide, cyanide ion,
<br />simple cyanides, complex cyanides, nitriles, cyano-
<br />gens,and total cyanide. The most common forms of
<br />cyanide in the environment are free cyanide,
<br />metaliocyanide complexes, and synthetic nitriles.
<br />A brief description of each cyanide species follows
<br />(Smith et al. 1978, 1979; Towill et al. 1978;
<br />Egekeze and Oehme 1980; EPA 1980, 1989; Davis
<br />1981; Leduc 1981, 1984; Leduc et a1.1982; Simovic
<br />and Snodgrass 1985; Ballantyne 1987a; Homan
<br />1987; Marrs and Ballantyne 1987).
<br />Free cyanide is the primary toxic agent in the
<br />aquatic environment. Free cyanide refers to the
<br />sum of molecular HCN and the cyanide anion
<br />(CN-), regardless of origin. In aqueous solution
<br />with pH 9.2 and lower, the majority of the free cye-
<br />nide is in the form of molecular HCN. The chemical
<br />names for HCN include hydrogen cyanide,
<br />hydrocyanic acid, cyanohydri acid, and prussic
<br />acid. Hydrogen cyanide (Table 1) is a colorless,
<br />flammable liquid or gas that boils at 25.7° C and
<br />freezes at -13.2° C. The gas rarely occurs in na-
<br />ture, is lighter than air, and diffuses rapidly; it is
<br />usually prepared commercially frown ammonia and
<br />methane at elevated temperatures with a plati-
<br />numcatalyst. Itismiscible with water and alcohol,
<br />but is only slightly soluble in ether. In water, HCN
<br />is a weak acid with the ratio of HCN to CN' about
<br />100 at pH 7.2, 10 at pH 8.2, and 1 at pH 9.2. HCN
<br />can dissociate into H' and CN-. Cyanide ion, or free
<br />cyanide ion, refers to the anion CN" derived from
<br />hydrocyanic acid in solution, in equilibrium with
<br />simple or complexed cyanide molecules. Cyanide
<br />ions resemble halide ions in several ways and are
<br />sometimes referred to as "pseudohaline" ions. For
<br />example, silver cyanide is almo$t insoluble in
<br />water, as are silver halides. Cyanide ions also form
<br />stable complexes with many metals.
<br />Simple cyanides typically refer to alkali
<br />water-soluble salts, such as NaCN, KCN, Ca(CN)2,
<br />and Hg(CN)2, but also include several cyanide
<br />salts of alkali, alkaline earth, or heavy metals, that
<br />is, Zn(CNh, Cd(CN),, Ni(CNs), and AgCN, of vary-
<br />ingdegrees ofsolubility. Inwater, NaCN and KCN
<br />will completely dissociate to give free cyanide. All
<br />simple cyanides ionize in water to release cyanide
<br />ion which, depending on pH, will form hydrocyanic
<br />acid. For sodium cyanide, the reaction proceeds as
<br />follows:
<br />(I) NaCN ~ Na' + CN'
<br />(2) CN-+ HOH ~ HCN + OH-
<br />Increased pH will maintain a larger fraction of
<br />the cyanide as CN-,and acidification will cause the
<br />reverse. At pH 7, about 99%of the free cyanide is in
<br />the form of HCN, whereas at pH 9.3 HCN com-
<br />poses 50%. Since HCN is extremely water soluble
<br />Table 1. Some properties of potassium cyanide, hydrogen ryanide, and sodium cyanide(from EPA
<br />7989).
<br />
<br />Property Potassium
<br />cyanide Hydrogen
<br />cyanide Sodium
<br />cyanide
<br />CAS number 151-50-8 74-90-8 149-33-9
<br />Chemical formula KCN HCN N$CN
<br />Molecular weight 65.12 27.03 49.01
<br />Physical state Solid Gas or liquid Solid
<br />Boiling point (° C) - 25.7 1,49$
<br />Melting point (° C) 634.5 -13.21 56$.7
<br />Specific gravity 1.5 0.7 (liquidl 1.6
<br />Solubility in water (g/L) 716 at 20° C Miscible 480 at 10° C
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